1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Streaming media services (e.g. music, video) over wireless communication networks have been gaining in popularity, and are likely to become commercially important to wireless service providers in the near future. A major impediment to their success is the often poor and/or unreliable audio or video quality associated with these services. Packets transmitted through the wireless communication network may be lost, delayed, or experience jitter. For example, signal strength fluctuations due to environmental changes and the need to share the wireless access medium among multiple users lead to significant fluctuations in the rate at which packets carrying a media stream are delivered to mobile units and/or the applications running on the mobile unit such as a media player. Packets may also be lost as they traverse the air interface from the media server to the client, which may cause interruptions in the media service and/or degraded quality of the media service. Conventional media sessions attempt to reduce the effects of lost packets, delayed packets, and/or jitter by buffering the received data stream.
FIG. 1 conceptually illustrates one exemplary embodiment of a conventional system 100 for streaming media over a wireless network. The radio links 102 between the base stations 107 and mobile clients 110 constitute the only wireless segment of the system 100. Although the system 100 as a whole comprises wired as well as wireless segments, it is conventionally referred to as a wireless network 100. A core network 105 lies between the Gateway GPRS Support Node (GGSN) 120 and the media server 115. The network segment between the GGSN 120 and the mobile client 110 (which typically includes GGSN 120 and mobile client 110) is conventionally referred to as the wireless access network 125. In the illustrated embodiment, the wireless access network 125 is based on the Universal Mobile Telecommunications System (UMTS) (3GPP) standard. However, the wireless access network 125 may also operate according to other wireless networking technologies and standards, e.g., cdma2000 High Rate Packet Data (HRPD) or IEEE 802.16e/WiMAX. In the case of cdma2000 HRPD, for instance, system 100 would appear identical to that in FIG. 1, except that the node pair, Serving GPRS Support Node (SGSN) 103 and Gateway GPRS Support Node (GGSN) 120, is replaced by a single entity known as the Packet Data Serving Node (PDSN). Furthermore, although a hierarchical architecture is illustrated, the wireless network 100 may also implement flat or distributed Internet Protocol (flat-IP) based architectures where Layer 3 routing (i.e., IP routing) and control functions relating to the wireless access network 125 are performed by a base station router that merges the base-station 107, radio network controller (RNC) 130, SGSN 103 and GGSN 120 into a single entity.
In the illustrated embodiment, a mobile client 110 may initiate a streaming video session with a media server 115 over the wireless network 100. For example, the client 110 may request a streaming video session by sending an RTSP message to the server 115. To initiate a media session, the mobile client 110 exchanges signaling messages with the media server 115 to establish a streaming media session and negotiate session parameters, e.g. the bit-rate at which the media is to be streamed. The mobile client 110 also exchanges lower-layer signaling messages with the RNC 130, the SGSN 103, and the GGSN 120 to establish a radio access bearer channel. The radio access bearer channels are typically configured to maintain desired Quality-of-Service (QoS) characteristics, e.g. if best-effort bearer service is deemed inadequate. Once the radio access bearer channel is established and the streaming media session is set up, the media server 115 transmits packets carrying the media to the mobile client 110, via the GGSN 120, the SGSN 103, the RNC 130, and the base station 107. The mobile client 110 sends periodic feedback messages along the reverse path from the base station 107 to the RNC 130, SGSN 103, GGSN 120, and media server 115. Owing to uplink bandwidth limitations in wireless access networks, the uplink feedback messages are transmitted relatively infrequently, e.g. once every 3-4 seconds.
Packets carrying the media and feedback messages transmitted by the mobile client 110 are carried transparently by the network elements. Thus, the signaling (in the form of feedback messages from the mobile client 110) that helps the media server 115 make control decisions (such as changing transmission rate or content rate) is essentially end-to-end, with no intervention by the network elements. For example, the media server 115 may be responsible for content rate control. Conventional media servers 115 perform content rate control by estimating buffer levels at the mobile client 110 based upon the number of packets transmitted by the media server 115 and an estimate of the play out time at the mobile client 110 that is determined using the end-to-end feedback from the mobile client 110. The media server 115 may also transmit some control/signaling messages to the mobile client 110 on a periodic basis. These messages, such as “server reports” are also carried transparently by the network elements. The media server's control decisions are therefore based on the rather infrequent feedback received from the mobile client 110, which does not have direct knowledge of the channel conditions. Consequently, the media server 115 cannot make timely decisions to avoid packet losses or prevent rebuffering events that are detrimental to the quality of the streaming media service.